1. Field of the Invention
This invention relates to a method for compressing a dynamic range of an image, with which an original image signal representing an original image is processed, and a processed image signal representing an image having a narrower dynamic range than the original image is thereby generated.
2. Description of the Prior Art
Techniques for reading out a recorded image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored therein during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In the image recording and reproducing systems, with which image signals are generated and visible images are reproduced from the image signals, it often occurs that portions of an image, which are to be used and therefore are required to have an appropriate image density in the reproduced image, have image density levels ranging widely from a low density to a high density. Also, it often occurs that what portions of an image having a wide range of image density are to be used and therefore are required to have an appropriate image density in the reproduced image. In such cases, the image signal representing the original image is processed such that the high-density parts of the original image may have an appropriate level of image density in the reproduced image. Also, the image signal representing the original image is processed such that the low-density parts of the original image may have an appropriate level of image density in the reproduced image. Thereafter, both of the images reproduced from the image signals, which have thus been processed in different ways, are displayed side by side on a single display device.
However, if a plurality of images are displayed side by side on a single display device, problems will occur in that the images inevitably become small in size and therefore hard to observe.
In order that parts of an image covering as wide a range of image density as possible can be used, the level of contrast of the parts of the image having a high or low image density or the level of contrast of the whole image has heretofore been rendered low such that the difference between the highest image density and the lowest image density is small, i.e. such that the dynamic range of the image is narrow.
However, if the level of contrast is rendered low, problems will occur in that details of the image information in the image region, at which the level of contrast has been lowered, becomes hard to observe.
How the problems described above occur will be described hereinbelow.
FIG. 8 is a graph showing an example of how the values of the image signal components of an original image signal Sorg are distributed, which image signal components represent picture elements located along a certain direction (the direction indicated by the arrow x) on an original image. As a whole, the values of the image signal components of the original image signal Sorg are distributed in a step-like pattern along the direction indicated by the arrow x (i.e. the image density of the original image changes step-wise along the direction indicated by the arrow x). Each step part of the distribution of the image density includes a fine change having a comparatively high spatial frequency.
FIG. 9A is a graph showing an example of characteristic with which an original image signal Sorg representing an image is transformed into an image signal Sorg' such that the dynamic range of the image may be compressed by lowering the level of contrast of the parts having a high image density. FIG. 9B is a graph showing how the values of the image signal components of the image signal Sorg' resulting from the transformation with the characteristics illustrated in FIG. 9A are distributed, which image signal components represent picture elements located along the direction indicated by the arrow x on the image.
In this example, the original image signal Sorg having values shown in FIG. 8 is transformed into the image signal Sorg' having values lying on the line A shown in FIG. 9A. As a result, as illustrated in FIG. 9B, the level of image density in the part having a high image density become low. Also, the level of contrast of the fine image structures represented by the fine change in each step part of the distribution of the image density, which step part falls within the high density region, becomes low. Therefore, the fine image structures having a high image density, which image structures are to be used and therefore are required to have good image quality in the reproduced image, become very hard to observe.
FIG. 10A is a graph showing an example of characteristics with which an original image signal Sorg representing an image is transformed into an image signal Sorg' such that the dynamic range of the image may be compressed by lowering the level of contrast of part having a low image density. FIG. 10B is a graph showing how the values of the image signal components of the image signal Sorg' resulting from the transformation with the characteristics illustrated in FIG. 10A are distributed, which image signal components represent picture elements located along the direction indicated by the arrow x on the image.
In this example, the original image signal Sorg having values shown in FIG. 8 is transformed into the image signal Sorg' having values lying on the line B shown in FIG. 10A. In such cases, as illustrated in FIG. 10B, the fine image structures having a low image density become very hard to observe.